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On February 4, 2021, I was contacted by an Otago Daily Times reporter about the potential sources of the elevated lead levels in the Waikouaiti and Karitane drinking water supply.
Like many, at that time, I did not have any information on water sampling locations and lead levels at those locations. Since then, Dunedin City Council (DCC) has been providing online water sample graph data from Karitane Bowls, Waikouaiti Golf Club, Waikouaiti Golden Fleece Hotel-Motel, raw water reservoir and post-treated reservoir.
According to the above data, the golf club samples exceeded drinking water lead standard of 0.01 mg/L on four occasions — 0.03, 0.39, >0.01 and 0.01-0.02 mg/L between 31.7.2020 and 8.1.2021 — while the bowling club sample had >0.07 mg/L on 8.12.2020.
In addition to the above two sites, the raw water reservoir sample also had a single elevated level of >0.05 on 22.1.2021.
Having assessed the above data, am I in a position to predict the source or causes for the elevated levels? You have guessed it; I need more information.
At the time of writing this article, since July 31, 2020, there had been 27 samples/site from the golf club, bowling club and Golden Fleece, while 13 were taken from each two reservoirs.
To trace the lead source, it is critical to assess all samples found with detectable level of lead and compare for any trend between the sites and days of lead detected. In doing so, any lag between the sampling sites must also be considered, which may be more than 10km between some sampling sites. Since there is a similar trend of elevated and detectable levels of lead between most sampling sites, any sensible link between the sites can only be established if the same number of samples were obtained on the same days from all sites.
It is still not clear whether the Golden Fleece, golf club and bowling club water samples were collected from the tap or at the point of water entry into the above properties. Tap water is known to be contaminated by lead in the plumbing system. If lead is already present at any properties, any lead contamination in the water reticulation system can be amplified in the tap water sample owing to additional lead being introduced into the samples. Compared to the other two properties, the golf club had frequent and high detection of lead, on 19 occasions excluding the exceedance days, with the bowling club having six and Golden Fleece seven detections.
Globally, the potential sources of lead have been documented by researchers as diffused sources such as lead run-off from properties with high lead in soils (for example, from old paint flakes) entering the catchment, ashes or run-off from forest burning and historical discharge of lead shots from bird hunting and clay-bird shooting sites, or point sources such as any mining activities and unauthorised landfills, or in-pipe contamination from cast-iron pipe connections or on-site contamination from plumbing.
According the 2015-18 national estuarine and coastal sediment online data from Statistics NZ, of the 309 sites monitored, 12 sites exceeded the default guideline value (DGV) of 50 micrograms/gram for lead. The three sites sampled in the Waikouaiti estuary also had zinc, copper and cadmium below the respective DGV levels. From the above data I assume there has been no historical or regular point or diffused discharges causing lead or other heavy metal accumulation in the Waikouaiti River catchment.
If I assume there has been no new and significant discharge of lead into the above catchment post-2018 Statistics NZ data, the remaining lead sources must be from the water treatment site and the pipeline. Lead sources such as plumbing must also be isolated and ruled out.
Forest trees can sequester heavy metals such as lead from soils and redeposit as litter. Lead dispersion and subsequent water contamination caused by forest fires have been well documented. I understand there was a forest fire early last year near the water treatment plant. The burnt land parcel can be seen from the water treatment site photo published on February 5, 2021. Such a source should also be assessed and ruled out.
In 2019, Christchurch City Council collected 198 water samples from the Lyttelton and Sydenham area suspected to have the old pipelines with lead, and found only two samples exceeding lead standard. If the DCC pipelines are the source, lead detected in the reservoirs is unrelated to the above source. Sediment samples from the reservoirs could assist in eliminating the reservoirs as a potential source. The planned old pipeline replacement by DCC will also eliminate one of the potential sources.
To put things in perspective, while even the occasional elevated lead level in drinking water supply is alarming, as humans we have greater exposure to lead from other documented sources such as soils, old houses with lead paint, dust, food, old metal ornaments/jewellery, ayurvedic/folk medicines and occupational sources (people working with lead or lead sources). Unless soils or the atmosphere are heavily contaminated with lead, the extent of lead transfer to humans through food grown on the soils is low.
Owing to anthropogenic or geological processes, soils tend to have most heavy metals at varying levels. Thus, a normal rural soil may have 5-20 mg/kg of lead as background level. Even under indigenous forest cover, soil may contain 8 mg/kg of lead. Urban soils tend to have elevated levels of lead — levels such 200 mg/kg may be detected frequently.
Plants including vegetables do absorb lead, mainly from roots and at times through leaves. Different plant species possess different rate of uptake depending on soil conditions, bioavailability of lead and where they accumulate lead.
According to our 2011 National Environmental Standard Assessing and Managing Contaminants in Soil to Protect Human Health, soil lead threshold was set at 160 mg/kg for rural residential/lifestyle blocks with 25% home grown vegetable consumed. Using the recent largest lead spike in the DCC water supply, a one-off vegetable garden irrigation of 10mm water (10 L/square metre) would have deposited 3.9mg lead at 0.39 mg/L lead level in water. Ten such irrigations would have resulted in 39mg lead discharged per square metre of soil, which would have resulted in a soil lead level of 0.2 mg/kg in the top 15cm soil layer. Thus, the risk of high lead accumulation in soil by irrigation, and the subsequent uptake by vegetables — particularly with occasional lead spikes — is low.
I hope the causes for the lead contamination are isolated and fixed soon or the lead levels continue to stay below the threshold level. The recent events have been a New Zealand-wide wake-up call to maintain good water catchment and water take, treatment and delivery systems. No matter how well the local councils treat and supply lead-free drinking water, having plumbing fittings with lead can contaminate water. Stringent standards must be imposed by the central government on imported taps and plumbing fittings to ensure they are lead-free.
■ Dr Selva Selvarajah is the founder of Enviroknowledge Ltd, which trains environmental professionals.